Shirley Katz is not afraid to fight for her rights. Last week the schoolteacher, 44, went to court in her home town of Medford, Oregon, to protest at her working conditions. Specifically she is outraged she cannot carry a handgun into class. 'I know it is my right to carry that gun,' she said.

Katz was in court in the week that someone else took a gun to school in America. This time it was a pupil in Cleveland, Ohio. Asa Coon, 14, walked the corridors of his school, a gun in each hand, shooting two teachers and two students. Then he killed himself. Coon's attempted massacre made headlines. But a more bloody rampage, the murder of six young partygoers by Tyler Peterson, a policeman in Crandon, Wisconsin, got less attention, even in the New York Times - America's newspaper of record - which buried it deep inside the paper.

Guns, and the violence their possessors inflict, have never been more prevalent in America. Gun crime has risen steeply over the past three years. Despite the fact groups such as the National Rifle Association (NRA) consistently claim they are being victimised, there have probably never been so many guns or gun-owners in America - although no one can be sure, as no one keeps a reliable account. One federal study estimated there were 215 million guns, with about half of all US households owning one. Such a staggering number makes America's gun culture thoroughly mainstream.

An average of almost eight people aged under 19 are shot dead in America every day. In 2005 there were more than 14,000 gun murders in the US - with 400 of the victims children. There are 16,000 suicides by firearm and 650 fatal accidents in an average year. Since the killing of John F Kennedy in 1963, more Americans have died by American gunfire than perished on foreign battlefields in the whole of the 20th century.

Studies show that having a gun at home makes it six times more likely that an abused woman will be murdered. A gun in a US home is 22 times more likely to be used in an accidental shooting, a murder or a suicide than in self-defence against an attack. Yet despite those figures US gun culture is not retreating. It is growing. Take Katz's case in Oregon. She brought her cause to court under a state law that gives licensed gun-owners the right to bring a firearm to work: her school is her workplace. Such a debate would have been unthinkable a few decades ago. Now it is the battleground. 'Who would have thought a few years ago, we would even be having this conversation? But this won't stop here,' said Professor Brian Anse Patrick of the University of Toledo in Ohio. Needless to say, last week the judge sided with Katz and she won the first round of her case.

It is a nation awash with guns, from the suburbs to the inner cities and from the Midwest's farms to Manhattan's mansions. Gun-owning groups have been so successful in their cause that it no longer even seems strange to many Americans that Katz should want to go into an English class armed. 'They have made what was once unthinkable thinkable,' said Patrick, a liberal academic. He should know. He owns a gun himself. Even the US critics of gun culture are armed.

To look at the photographs in Kyle Cassidy's book Armed America is to glimpse a surreal world. Or at least it seems that way to many non-Americans. Cassidy spent two years taking portrait shots of gun owners and their weapons across the US.

The result is a disturbing tableau of happy families, often with pets and toddlers, posing with pistols, assault rifles and the sort of heavy machine-guns usually associated with a warzone. 'By the end I had seen so many guns and I knew so much about guns that it no longer seemed unusual,' Cassidy said. He keeps his in a gun safe in his home in Philadelphia. 'This turned into a project not about guns but about a diverse group of people,' he said.

At the cutting edge of weapon culture remains the gun lobby and its most vocal advocate, the NRA. Founded in the 19th century by ex-Civil War army officers dismayed at their troops' lack of marksmanship, the NRA has transformed into the most effective lobbying group in Washington DC. It has scores of lobbyists, millions of dollars in funds and more than three million members. It is highly organised and its huge membership is highly motivated and activist. They can have a huge influence on politics.

In 2000 Vice-President Al Gore supported stricter background checks for gun-buyers and the NRA organised against him, describing the election as the most important since the Civil War. It spent $20m against Gore in an election ending in a razor's edge result. Its influence was especially felt in Gore's home state of Tennessee, which he narrowly lost to NRA gloating. 'Their vote can select the President. They don't get to pick who goes to the White House. But they can tip the balance,' said Patrick.

Democrats have learnt that lesson now. Many shy away from gun control issues, wary of taking on such a vociferous lobby group. In the 2006 mid-term elections the NRA was able to back a historically high 58 Democrats running for office. Every one of them went on to win. Such influence over the past three decades has seen the NRA fight a successful campaign against new gun laws. It has in fact loosened regulations, spreading the ability to legally carry concealed weapons across 39 states. And this has all been done in the face of a fight from anti-gun groups, backed by much of the mainstream media. 'Politicians are so afraid of the gun lobby. They run scared of it,' said Joan Burbick, author of the book Gun Show Nation

But the key question is not about the number of guns in America; it is about why people are armed. For many gun-owners, and a few sociologists, the reason lies in America's past. The frontier society, they say, was populated by gun-wielding settlers who used weapons to feed their families and ward off hostile bandits and Indians. America was thus born with a gun in its hand. Unfortunately much of this history is simply myth. The vast majority of settlers were farmers, not fighters. The task of killing Indians was left to the military and - most effectively - European diseases. Guns in colonial times were much rarer than often thought, not least because they were so expensive that few settlers could afford them. Indeed one study of early gun homicides showed that a musket was as likely to be used as club to beat someone to death as actually fired.

But many Americans believe the myth. The role of the gun is now enshrined in mass popular culture and has huge patriotic significance. Hence the fact that gun ownership is still a constitutional right, in case America is ever invaded and needs to form a popular militia (as hard as that event might be to imagine). It also explains why guns are so prevalent in Hollywood. Currently playing in US cinemas is the Jodie Foster film The Brave One, a classic vigilante movie of the wronged woman turning to the power of the pistol to murder the criminals who killed her boyfriend. Foster's character is played as undeniably heroic. 'There is a fascination with guns in our culture. All you need to make a movie is a girl and a gun,' said Cassidy.

But this worship of the gun in many ways springs from economics and social problems, not the historic frontier. It took mass production and mass marketing to really popularise firearms. The Civil War saw mass arms manufacturing explode in America, including making 200,000 Colt .44 pistols alone. It saw guns become familiar and cheaper for millions of Americans. The later 19th century saw gun companies using marketing techniques to sell their weapons, often invoking invented frontier imagery to do so. That carries on today. There are more than 2,000 gun shows each year, selling hundreds of thousands of guns. It is big business and business needs to sell more and more guns to keep itself profitable. 'They will do anything to sell guns,' said Burbick.

But there are deeper issues at work too. The gun lobby's main argument is that guns protect their owners. They deter criminals and attackers whom - the gun lobby points out helpfully - are often armed themselves. Some surveys estimate there are more than two million 'defensive' uses of firearms each year. But others say that this argument is a shield, using guns as a way of deflecting harder arguments about how crime is caused by economics, poverty and racism. 'The argument over guns redefines a lot of social issues as simple aspects of crime,' said Burbick. She argues that a way to make Americans feel safer from crime is not to arm them with guns but to tackle the causes of crime: urban poverty, joblessness, drug addiction and racial divisions. 'We have to take back the language of human security. To talk about solving those social issues in terms of safety, not just letting the gun lobby control that language,' she said.

It is a powerful argument. Critics of America's gun culture often point to other nations with high levels of gun ownership - such as Canada and Switzerland - but much lower levels of violent crime. The fact is that America itself is equally divided. Patrick lives in a quiet, rural part of Michigan just across the state line from Ohio and the town of Toledo where he works. 'I would be amazed if anyone within four miles of me did not have a gun,' he said 'But our homicide rate is zero.'

Then look at where Cassidy lives. He has an apartment in Philadelphia, a city that is just as flooded with guns as Patrick's rural idyll, but also suffers from inner-city social ills. It has a stratospheric murder rate. 'There is a murder here every day. This is something that America has to come to terms with,' he said. Yet the differences do not lie with the simple existence of guns. Both places are full of them. They lie with the root causes of crime and violence, such as poverty and drugs, that blight many big cities. Guns seem neither to be totally the problem and certainly not the solution.

However, that is a debate few in America are having. In the meantime, the gun culture is so firmly entrenched and society so full of guns that there is little prospect of it retreating. Even those who advocate much tighter laws have long accepted defeat of the ideal of creating a society where guns are rare in public life, or even completely absent. 'That notion is absurd. There is no way to de-gun America,' said Patrick.

To cap a grim week, as Katz was winning her court battle in Oregon police in Pennsylvania were giving details of a raid on the home of a teenager who was plotting to attack a school. They found seven home-made grenades and an assault rifle. His mother had bought it for him at a gun show. The boy was just 14.

America's worst shooting sprees

Virginia Tech Seung-Hui Cho a Korean American, was a loner who scared classmates. In April he killed 32 students and staff, then himself, at Virginia Tech, the worst US school shooting.

Amish killings On a Monday morning in October 2006, truck driver Charles Roberts opened fire in a school in Paradise, Pennsylvania. He killed five children, then shot himself.

Columbine Colorado misfits Dylan Klebold and Eric Harris assaulted their high school, Columbine, in April 1999, killing 12 students and a teacher. They then committed suicide.

Luby's massacre In October, 1991 George Hennard drove a truck into Luby's Cafe in Killeen, Texas, shot dead 23 people and injured another 20 before shooting himself.

'Going postal' Patrick Sherrill, an Oklahoma postal employee, took a gun to work in August 1986, shot 14 staff, then killed himself.

McDonald's massacre In January 1984 in San Ysidro, California, James Huberty killed 21 with an Uzi and other guns at a McDonald's. He was killed by a Swat sniper.

Texas tower shooting In 1966 Charles Whitman murdered his wife and mother, then climbed a University of Texas observation tower in Austin. He shot and killed 14 people before police shot him.